Involvement of the nitric oxide pathway in the anticonvulsant effect of tramadol on pentylenetetrazole-induced seizures in mice

In the present study, the effects of tramadol on pentylenetetrazole (PTZ)-induced seizures and involvement of nitric oxide (NO) were assessed in mice. To determine the threshold for clonic seizures, PTZ was administered intravenously. Tramadol was administered intraperitoneally (0.5-50mg/kg) 30 minutes prior to induction of seizures. The effects of the nitric oxide synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME; 0.5, 1, 5, and 10mg/kg), the nitric oxide precursor L-arginine (10, 30, and 60 mg/kg), and the nonspecific opioid receptor antagonist naloxone (0.1, 0.5, 1, and 5mg/kg) on the anticonvulsant effect of tramadol were investigated. Administration of tramadol (1mg/kg) increased the threshold for seizures induced with PTZ in a monophasic, dose-independent, and time-dependent manner. Acute administration of L-NAME (5 and 10mg/kg) inhibited the anticonvulsant effect of tramadol (1mg/kg), whereas L-arginine, in the noneffective dose range (30 and 60 mg/kg), potentiated the seizure threshold when co-administered with a subeffective dose of tramadol (0.5mg/kg). Naloxone partially and dose-independently antagonized the anticonvulsant effect of tramadol (1mg/kg). These results indicate that the anticonvulsant effect of tramadol is mediated by the nitric oxide pathway and also by classic opioid receptors.     

Possible involvement of PPAR-gamma receptor and nitric oxide pathway in the anticonvulsant effect of acute pioglitazone on pentylenetetrazole-induced seizures in mice

Besides the receptor-mediated effects of pioglitazone, the involvement of nitric oxide (NO) has been previously demonstrated in some pioglitazone-induced central and peripheral effects. In the present study, the effects of acutely administered pioglitazone on pentylenetetrazole (PTZ)-induced seizures and involvement of NO were evaluated in mice. To determine the threshold for clonic seizures, PTZ was administered intravenously. A single dose of pioglitazone (10, 20, 40 and 80 mg/kg, p.o.) was administered either 2 or 4h prior to induction of seizures. For determination of possible role of peroxisome proliferator activated receptor gamma (PPAR-γ) and nitric oxide pathway in this effect, the effects of a PPAR-γ antagonist, GW9662 (2 mg/kg); a non-specific nitric oxide synthase (NOS) inhibitor, NG-nitro-L-arginine methyl ester (L-NAME; 0.3, 1, 3, and 10 mg/kg); a specific iNOS inhibitor, aminoguanidine (100mg/kg, i.p.) or a nitric oxide precursor, L-arginine (30, 60, 100 and 200mg/kg, i.p.); each administered 15 min prior to pioglitazone, were investigated on the anticonvulsant effect of this drug. Administration of pioglitazone (40 and 80 mg/kg) increased the threshold of PTZ-induced seizure in a dose-dependent, and time-dependent manner. GW9662 reversed the anticonvulsant effect of pioglitazone (40 mg/kg). Acute administration of L-NAME (1, 3 and 10mg/kg) inhibited the anticonvulsant effect of pioglitazone (40 mg/kg), the same result was detected with aminoguanidine (100mg/kg); whereas L-arginine, in the noneffective dose (100mg/kg), potentiated the seizure threshold when co-administered with a subeffective dose of pioglitazone (20mg/kg). CONCLUSION: The present study demonstrates the anticonvulsant effect of acute pioglitazone on PTZ-induced seizures in mice. This effect was reversed by PPAR-γ antagonist, and both a specific- and a non-specific nitric oxide synthase inhibitors, and augmented by nitric oxide precursor, L-arginine. These results support that the anticonvulsant effect of pioglitazone is mediated through PPAR-γ receptor-mediated pathway and also, at least partly, through the nitric oxide pathway.     

Melatonin enhances the anticonvulsant and proconvulsant effects of morphine in mice: role for nitric oxide signaling pathway

Melatonin has different interactions with opioids including enhancing their analgesic effect and reversal of opioid tolerance and dependence. Opioids are known to exert dose-dependent anti- and proconvulsant effects in different experimental seizure paradigms. This study investigated the effect of melatonin on biphasic modulation of seizure susceptibility by morphine, in mouse model of pentylenetetrazole (PTZ)-induced clonic seizures. We further investigated the involvement of the nitric oxidergic pathway in this interaction, using a nitric oxide synthase inhibitor, NG-nitro-L-arginine-methyl-ester (L-NAME). Melatonin exerted anticonvulsant effect with doses as high as 40-80 mg/kg, but with a dose far bellow that amount (10 mg/kg), it potentiated both the anticonvulsant and proconvulsant effects of morphine on the PTZ-induced clonic seizures. Possible pharmacokinetic interaction of melatonin and morphine cannot be ruled out in the enhancement of two opposing effects of morphine on seizure threshold. L-NAME (1 mg/kg) reversed the anticonvulsant property of the combination of melatonin (10 mg/kg) plus morphine (0.5 mg/kg). Moreover, L-NAME (5 mg/kg) blocked the enhancing effect of melatonin (10 mg/kg) on proconvulsant activity of morphine (60 mg/kg). Our results indicate that co-administration of melatonin enhances both anti- and proconvulsant effects of morphine via a mechanism that may involve the nitric oxidergic pathway.     

Involvement of NMDA receptors and nitric oxide in the thermoregulatory effect of morphine in mice

Summary. Morphine has long been known to have potent effects on body temperature. It has been suggested that both N-methyl-D-aspartate (NMDA) receptors and nitric oxide (NO) pathway are involved in thermoregulation and also known to play important roles in some of morphine effects. The aim of this study was therefore to investigate the contribution of NMDA receptors and NO to the thermoregulatory effect of morphine. Morphine produced a hypothermic effect, especially at the dose of 10 mg/kg. Ketamine (5–40 mg/kg, i.p.) and N^G-nitro-L-arginine-methyl ester (L-NAME, 1–100 mg/kg, i.p.) also produced hypothermic effects with their higher doses. At doses which themselves produced no effect on colonic temperature in mice, both ketamine (10 mg/kg, i.p.) and L-NAME (10 mg/kg, i.p.) enhanced the hypothermic effect of morphine (10 mg/kg, i.p.). These results further support the relationship between NO and NMDA receptors and suggest a possible role of NMDA-NO pathway in the thermoregulatory effect of morphine. Received August 4, 1999; accepted October 6, 1999     

The role of nitric oxide in anticonvulsant and proconvulsant effects of morphine in mice

Acute subcutaneous administration of lower doses of morphine (0.5, 1 and 3 mg/kg) increase the threshold of seizures induced by pentylenetetrazole (PTZ) in mice, whereas higher doses of morphine (15, 30 and 60 mg/kg) have proconvulsant effects. The effect of systemic administration of nitric oxide synthase (NOS) inhibitors N(G)-nitro-L-arginine methyl ester (L-NAME) and N(G)-nitro-L-arginine (L-NNA) and nitric oxide synthase (NOS) L-arginine on biphasic effect of morphine was investigated. Acute administration of both L-NAME (1, 3 and 10 mg/kg) and L-NNA (1 and 10 mg/kg) as well as chronic pretreatment with L-NAME (1 and 10 mg/kg, 4 days) dose-dependently inhibited both the anticonvulsant and proconvulsant effects of morphine (1 and 30 mg/kg, respectively). The inhibition was complete for anticonvulsant effect while partial for proconvulsant effect. L-arginine at doses that did not affect seizure threshold per se (acute, 30 and 60 mg/kg; chronic, 60 mg/kg) potentiated both anticonvulsant and proconvulsant properties of less potent doses of morphine (0.5 and 15 mg/kg, respectively). The L-arginine induced potentiation of both phases of morphine effect was blocked by L-NAME (0.5-30 mg/kg). Moreover, low and per se non-effective doses of naloxone (0.1 mg/kg) and L-NAME (0.3, 0.5 or 1 mg/kg) showed additive effects in inhibiting both phases of morphine effects. These results support the involvement of L-arginine/nitric oxide pathway in the modulation of seizure threshold by morphine.     

Involvement of the NMDA receptor/nitric oxide signal pathway in platelet-activating factor-induced neurotoxicity

Platelet-activating factor (PAF), a bioactive phospholipid implicated in neuronal excitotoxic death, augments the presynaptic release of glutamate. Excessive activation of postsynaptic glutamate receptors and subsequent downstream signals leads to excitotoxicity. The present study proposed that the NMDA receptor/nitric oxide (NO) signal pathway might be involved in PAF-induced neurotoxicity. After the cultured neurons were exposed to PAF for 24 h the percentage of neuronal death increased in a dose-dependent manner. The PAF effects were significantly prevented not only by BN52021, a PAF antagonist, but also by MK-801, an NMDA antagonist, and L-NAME, an NO synthase (NOS) inhibitor. Moreover, the increases in NOS activity and neuronal NOS expression induced by chronic exposure of the cultured neurons to PAF were dramatically blocked by BN52021 and MK-801, respectively. Our findings suggest that the NMDA receptor/NO signaling pathway might contribute to the pathological mechanism of cell death triggered via PAF receptor activation.     

Involvement of nitric oxide in phencyclidine-induced place aversion and preference in mice

The present study investigated the involvement of nitric oxide (NO) in phencyclidine (PCP)-induced place aversion and preference in the place conditioning paradigm. PCP-induced place aversion in naive mice was dose-dependently attenuated by administration of N(G)-nitro-L-arginine methyl ester (L-NAME), a NO synthase (NOS) inhibitor, during the conditioning. The NOS activity and dopamine (DA) turnover in the hippocampus in mice showing PCP-induced place aversion were decreased, such changes being restored by administration of L-NAME during the conditioning. On the other hand, PCP-induced place preference in mice pretreated with PCP for 28 days was not attenuated by administration of L-NAME during the conditioning. Although NOS activity was not changed, the DA turnover in the cerebral cortex was increased in mice showing PCP-induced place preference. In mice pretreated with L-NAME and PCP for 28 days before the place conditioning paradigm, PCP neither induced place preference, nor changed the NOS activity or DA turnover. These results suggest that NO is involved in the acquisition of PCP-induced aversive effects, and in the development of PCP-induced preferred effects. Further, the functional change of the DAergic neuronal system mediated by NO in the hippocampus and cerebral cortex may be necessary for the expression of aversive effects and development of preferred effects, respectively, induced by PCP.     

Anticonvulsant effect of dextrometrophan on pentylenetetrazole-induced seizures in mice: Involvement of nitric oxide and N-methyl-d-aspartate receptors

Dextrometrophan (DM), widely used as an antitussive, has recently generated interest as an anticonvulsant drug. Some effects of dextrometrophan are associated with alterations in several pathways, such as inhibition of nitric oxide synthase (NOS) enzyme and N-methyl d-aspartate (NMDA) receptors. In this study, we aimed to investigate the anticonvulsant effect of acute administration of dextrometrophan on pentylenetetrazole (PTZ)-induced seizures and the probable involvement of the nitric oxide (NO) pathway and NMDA receptors in this effect. For this purpose, seizures were induced by intravenous PTZ infusion. All drugs were administrated by intraperitoneal (i.p.) route before PTZ injection. Our results demonstrate that acute DM treatment (10–100 mg/kg) increased the seizure threshold. In addition, the nonselective NOS inhibitor L-NAME (10 mg/kg) and the neural NOS inhibitor, 7-nitroindazole (40 mg/kg), at doses that had no effect on seizure threshold, augmented the anticonvulsant effect of DM (3 mg/kg), while the inducible NOS inhibitor, aminoguanidine (100 mg/kg), did not affect the anticonvulsant effect of DM. Moreover, the NOS substrate l-arginine (60 mg/kg) blunted the anticonvulsant effect of DM (100 mg/kg). Also, NMDA antagonists, ketamine (0.5 mg/kg) and MK-801 (0.05 mg/kg), augmented the anticonvulsant effect of DM (3 mg/kg). In conclusion, we demonstrated that the anticonvulsant effect of DM is mediated by a decline in neural nitric oxide activity and inhibition of NMDA receptors.     

Involvement of the nitric oxide pathway in synaptic dysfunction following amyloid elevation in Alzheimer's disease

Amyloid-beta (Abeta), a peptide thought to play a crucial role in Alzheimer's disease (AD), has attracted scientific interest with the aim of characterizing the mechanisms by which it is involved in AD pathogenesis. Abeta has been found to markedly impair hippocampal long-term potentiation (LTP), a widely studied cellular model of synaptic plasticity that is thought to underlie learning and memory. The overall purpose of this review is to define the role of the nitric oxide (NO)/cGMP/cAMP-regulatory element binding (CREB) pathway in beta-amyloid-induced changes of basal neurotransmission and synaptic plasticity in the hippocampus, a structure within the temporal lobe of the brain critical for memory storage.     

Involvement of nitric oxide and nitric oxide synthase activity in anticonvulsive action

The anticonvulsant drug Diazepam (DIA-2 mg/kg b. wt), the nitric oxide (NO) donor L-Arginine (L-Arg-2000 mg/kg b. wt) and the putative nitric oxide synthase (NOS) inhibitor N(G)-Nitro-L-Arginine methyl ester (L-NAME-50 mg/kg b. wt) were used to determine the role of endogenous NO on convulsions induced by picrotoxin (PCT-5 mg/kg b. wt) in rats. Rats given a convulsant dose of PCT (5 mg/kg b. wt) had convulsion and it suppresses the NOS activity and NO concentration in brain regions. The anticonvulsant L-Arg alone significantly increases the NO concentration and NOS activity in brain regions, but not diazepam. Whereas DIA, along with L-Arg, enhances the NO and NOS activity when compared to L-Arg alone. The combination of both OIA and L-Arg completely suppressed the convulsions. L-NAME alone had no effect to produce convulsions but it completely decreased NO concentration and NOS activity and potentiated the PCT convulsions. This was reverted by pre- and post treatment of DIA plus L-Arg indicating, the increased NO concentration and NOS activity in brain regions suppresses convulsions.     

Antidepressant-like effect of pioglitazone in the forced swimming test in mice: the role of PPAR-gamma receptor and nitric oxide pathway

In this study, the potential antidepressant-like effects of pioglitazone and the possible involvement of peroxisome proliferator-activated receptor gamma (PPARγ) and nitric oxide system in antidepressant effects of pioglitazone were determined using forced swimming test (FST) in mice.

Method

After assessment of locomotor activity in open-field test, mice were forced to swim individually and the immobility time of the last 4 min was evaluated. Pioglitazone was administered orally with doses (5, 10, 20 and 30 mg/kg) 2 and 4 h before FST. To assess the involvement of PPARγ in the possible antidepressant effect of pioglitazone, GW9662, a PPARγ antagonist (2 mg/kg) was administered before pioglitazone (20 mg/kg). For determination of possible role of nitric oxide pathway in this effect, a non-specific NOS inhibitor, Nω-nitro-l-arginine methyl ester (l-NAME, 10 mg/kg, i.p.), a specific iNOS inhibitor, aminoguanidine (50 mg/kg, i.p.), or a NO precursor, l-arginine (750 mg/kg, i.p.) was co-administered with pioglitazone, either 2 or 4 h before FST.

Results

The immobility time significantly decreased after pioglitazone administration (20 and 30 mg/kg). GW-9662 significantly reversed antidepressant effect of pioglitazone administered 2 and 4 h prior to FST. Co-administration of non-effective doses of pioglitazone and l-NAME revealed antidepressant-like effect in FST; while, co-administration of non-effective doses of aminoguanidine and pioglitazone did not affect the immobility time. l-Arginine also reversed the antidepressant-like effect of pioglitazone.

Conclusion

The antidepressant-like effect of pioglitazone on mice in the FST is mediated at least in part through PPARγ receptors and nitric oxide pathway.     

Involvement of NMDA receptors and L-arginine-nitric oxide pathway in the antidepressant-like effects of zinc in mice

This study investigated the involvement of NMDA receptors and the L-arginine-nitric oxide (NO) pathway in the antidepressant-like effects of zinc in the forced swimming test (FST). The immobility times in the FST and in the tail suspension test (TST) were reduced by zinc chloride (ZnCl(2), 30 and 10-30 mg/kg intraperitoneal (i.p.), respectively). The doses active in the FST and TST reduced locomotor activity in an open-field. The antidepressant-like effect of ZnCl(2) in the FST was prevented by pre-treatment of animals with guanosine 5'-monophosphate (GMP), ascorbic acid, L-arginine, or S-nitroso-N-acetyl-penicillamine (SNAP), but not with D-arginine, administered at doses that per se produced no anti-immobility effect. The immobility time of mice treated with ZnCl(2)+MK-801 was not different from the result obtained with ZnCl(2) or MK-801 alone, but ZnCl(2)+imipramine had a greater effect in the FST than administration of either drug alone. Pre-treatment of animals with a sub-threshold dose of ZnCl(2) prevented the anti-immobility effect of MK-801, ketamine, GMP, L-arginine or N(G)-nitro-L-arginine (L-NNA), but did not alter the effect of imipramine or fluoxetine. Taken together, the results demonstrate that zinc produced an antidepressant-like effect that seems to be mediated through its interaction with NMDA receptors and the L-arginine-NO pathway.     

Involvement of nitric oxide-cGMP pathway in the antidepressant-like effect of ascorbic acid in the tail suspension test

Clinical and preclinical data reported that ascorbic acid has antidepressant properties. The present study was designed to investigate the participation of l-arginine-NO-cGMP pathway in the antidepressant-like effect of ascorbic acid in the tail suspension test (TST) in mice. The antidepressant-like effect of ascorbic acid (1 mg/kg, p.o.) in the TST was prevented by the pre-treatment of mice with NMDA (0.1 pmol/site, i.c.v.), l-arginine (750 mg/kg, i.p., a substrate for nitric oxide synthase) or sildenafil (5 mg/kg, i.p., a phosphodiesterase 5 inhibitor). The administration of MK-801 (0.001 mg/kg, i.p), 7-nitroindazole (25 mg/kg, i.p., a neuronal nitric oxide synthase inhibitor) or ODQ (30 pmol/site i.c.v., a soluble guanylate cyclase inhibitor) in combination with a sub-effective dose of ascorbic acid (0.1 mg/kg, p.o.) reduced the immobility time in the TST test when compared with either drug alone. None of the results in the TST appears to be due to a nonspecific locomotor effect. Our findings provide evidence that the effect of ascorbic acid in the TST involve an interaction with NMDA receptors and l-arginine-NO-cGMP pathway, contributing to the understanding of the mechanisms underlying the antidepressant-like effect of this vitamin.     

Involvement of the nitric oxide-cyclic GMP pathway and neuronal nitric oxide synthase in ATP-induced Ca2+ signalling in cochlear inner hair cells

We recently demonstrated that extracellular adenosine 5'-triphosphate (ATP) induced nitric oxide (NO) production in the inner hair cells (IHCs) of the guinea pig cochlea, which inhibited the ATP-induced increase in the intracellular Ca(2+) concentrations ([Ca(2+)](i)) by a feedback mechanism [Shen, J., Harada, N. & Yamashita, T. (2003) Neurosci. Lett., 337, 135-138]. We herein investigated the role of the NO-cGMP pathway and neuronal NO synthase (nNOS) in the ATP-induced Ca(2+) signalling in IHCs using the Ca(2+)-sensitive dye fura-2 and the NO-sensitive dye DAF-2. Fura-2 fluorescence-quenching experiments with Mn(2+) showed that ATP triggered a Mn(2+) influx. L-N(G)-nitroarginine methyl ester (L-NAME), a nonspecific NOS inhibitor, accelerated the ATP-induced Mn(2+) influx while S-nitroso-N-acetylpenicillamine (SNAP), a NO donor, suppressed it. 1H-[1,2,4]oxadiazole[4,3-a] quinoxalin-1-one, an inhibitor of guanylate cyclase, and KT5823, an inhibitor of cGMP-dependent protein kinase, enhanced the ATP-induced [Ca(2+)](i) increase. 8-Bromoguanosine-cGMP, a membrane-permeant analogue of cGMP mimicked the effects of SNAP. Moreover, the effects of 7-nitroindazole, a selective nNOS inhibitor, mimicked the effects of L-NAME regarding both the enhancement of the ATP-induced Ca(2+) response and the attenuation of NO production. Immunofluorescent staining of nNOS using a single IHC revealed that nNOS was distributed throughout the IHCs, but enriched in the apical region of the IHCs as shown by intense staining. In conclusion, the ATP-induced Ca(2+) influx may be the principal source for nNOS activity, which may interact with P2X receptors in the apical region of IHCs. Thereafter, NO can be produced and conversely inhibits the Ca(2+) influx via the NO-cGMP-PKG pathway by a feedback mechanism.     

Involvement of L-arginine-nitric oxide-cyclic guanosine monophosphate pathway in the antidepressant-like effect of venlafaxine in mice

The involvement of L-arginine-nitric oxide (NO)-cyclic guanosine monophosphate (cGMP) signaling pathway in the antidepressant action of venlafaxine (dual serotonin and norepinephrine reuptake inhibitor) was investigated in mice. The antidepressant activity was assessed in forced swim test (FST) behavioral paradigm. Total immobility time was registered during the period of 6 min. Venlafaxine produced dose-dependent (4-16 mg/kg, i.p.) reduction in immobility period. The antidepressant-like effect of venlafaxine (8 mg/kg, i.p.) was prevented by pretreatment with l-arginine (750 mg/kg, i.p.) [substrate for nitric oxide synthase (NOS)]. Pretreatment of mice with 7-nitroindazole (7-NI) (25 mg/kg, i.p.) [a specific neuronal nitric oxide synthase (nNOS) inhibitor] produced potentiation of the action of subeffective dose of venlafaxine (2 mg/kg, i.p.). In addition, treatment of mice with methylene blue (10 mg/kg, i.p.) [direct inhibitor of both nitric oxide synthase (NOS) and soluble guanylate cyclase (sGC)] potentiated the effect of venlafaxine (2 mg/kg, i.p.) in the FST. Furthermore, the reduction in the immobility time elicited by venlafaxine (8 mg/kg, i.p.) was also inhibited by pretreatment with sildenafil (5 mg/kg, i.p.) [phosphodiesterase 5 inhibitor]. The various modulators used in the study did not produce any changes in locomotor activity per se. The results demonstrated that the antidepressant-like effect of venlafaxine in the FST involved an interaction with the L-arginine-NO-cGMP pathway.     

Effects of d-penicillamine on pentylenetetrazole-induced seizures in mice: Involvement of nitric oxide/NMDA pathways

Besides the clinical applications of penicillamine, some reports show that use of d-penicillamine (d-pen) has been associated with adverse effects such as seizures. So, the purpose of this study was to evaluate the effects of d-pen on pentylenetetrazole (PTZ)-induced seizures in male NMRI mice. It also examined whether N-methyl-d-aspartate (NMDA) receptor/nitrergic system blockage was able to alter the probable effects of d-pen.Different doses of d-pen (0.1, 0.5, 1, 10, 100, 150, and 250 mg/kg) were administered intraperitoneally (i.p.) 90 min prior to induction of seizures. d-Penicillamine at a low dose (0.5 mg/kg, i.p.) had anticonvulsant effects, whereas at a high dose (250 mg/kg, i.p.), it was proconvulsant. Both anti- and proconvulsant effects of d-pen were blocked by a single dose of a nonspecific inhibitor of nitric oxide synthase (NOS), l-NAME (10 mg/kg, i.p.), and a single dose of a specific inhibitor of neuronal nitric oxide synthase (nNOS), 7-nitroindazole (30 mg/kg, i.p.). A selective inhibitor of iNOS, aminoguanidine (100 mg/kg, i.p.), had no effect on these activities. An NMDA receptor antagonist, MK-801 (0.05 mg/kg, i.p.), alters the anti- and proconvulsant effects of d-pen.The results of the present study showed that the nitric oxide system and NMDA receptors may contribute to the biphasic effects of d-pen, which remain to be clarified further.     

The effect of melatonin on protein oxidation and nitric oxide in the brain tissue of hypoxic neonatal rats

Melatonin is a potent antioxidant agent that can scavenge oxy- and nitroradicals generated under hypoxic conditions in the brain. In this study, we investigated the effect of melatonin on protein oxidation and nitric oxide (NO) during hypoxia. Seven-day-old Sprague-Dawley newborn rats were divided into three groups. Hypoxic (n=9) and melatonin (n=11) groups were subjected to 2h of hypoxic exposure (a humidity mixture of gases consisting of 92% nitrogen and 8% oxygen). Melatonin (at a dose of 10mg/kg) was administrated 30 min before the onset hypoxia and then at 24th and 48th hours after the end of the hypoxic exposure. Control (n=10) and hypoxic groups received the isotonic sodium chloride according to the same schedule. The brain tissue concentration of advanced oxidation protein products (AOPP) and protein thiol (P-SH) was used as an index of protein oxidation. In our study, although AOPP and NO increased significantly, the levels of P-SH decreased in the hypoxic group. The level of AOPP was declined by melatonin treatment. However, perturbed thiol status could not be recovered by melatonin treatment. There was no relationship between the levels of NO and protein oxidation markers. These results indicate that exogenous melatonin could prevent AOPP, but that it is inadequate in recovering perturbed thiol status. Therefore, melatonin alone was observed to be an incomplete treatment to prevent protein oxidation in hypoxia-induced brain damage.     

Involvement of the L-arginine/nitric oxide/cyclic guanosine monophosphate pathway in peripheral antinociception induced by N-palmitoyl-ethanolamine in rats

N-palmitoyl-ethanolamine (PEA) is an endogenous substance that was first identified in lipid tissue extracts. It has been classified as a CB(2) receptor agonist. Exogenous PEA has the potential to become a valid treatment for neuropathic and inflammatory pain. In spite of the well-demonstrated antiinflammatory properties of PEA, its involvement in controlling pain pathways remains poorly characterized. The participation of the L-arginine/nitric oxide (NO)/cyclic guanosine monophosphate (cGMP) pathway in peripheral antinociception has been established by our group to the μ-, κ- or δ-opioid receptor agonists, nonsteroidal analgesics, α(2C) -adrenoceptor agonists, and even nonpharmacological electroacupuncture. The aim of this study was to verify whether the peripheral antinociception effects of PEA involve the activation of this pathway. All drugs were locally administered to the right hind paw of male Wistar rats. The paw pressure test was used, with hyperalgesia induced by intraplantar injection of prostaglandin E(2) . PEA elicited a local peripheral antinociceptive effect that was antagonized by the nonselective NO synthase (NOS) inhibitor L-NOARG and the selective neuronal NOS (nNOS) inhibitor L-NPA. Selective inhibition of endothelial (eNOS) and inducible (iNOS) NOS via L-NIO and L-NIL, respectively, was ineffective at blocking the effects of a local PEA injection. In addition, the dosage of nitrite in the homogenized paw, as determined by colorimetric assay, indicated that exogenous PEA is able to induce NO release. The soluble guanylyl cyclase inhibitor ODQ antagonized the PEA effect, whereas the cGMP-phosphodiesterase inhibitor zaprinast potentiated the antinociceptive effect of low-dose PEA. This study provides evidence that PEA activates nNOS, thus initiating the NO/cGMP pathway and inducing peripheral antinociceptive effects.